The present invention relates generally to circuits for supplying drive current to lasers or other optical sources, and more particularly to input stage circuitry for a laser driver or other optical source driver.
Laser diodes and other types of semiconductor lasers are in widespread use as optical sources in high-speed optical data transmission applications. Laser diodes are particularly desirable in such applications due to their high optical output power and spectral purity. A laser driver circuit, also referred to herein as simply a xe2x80x9cdriver,xe2x80x9d is used to supply appropriate drive current to a semiconductor laser, so as to control the optical output signal between an xe2x80x9conxe2x80x9d state corresponding to a logic one level and an xe2x80x9coffxe2x80x9d state corresponding to a logic zero level, in accordance with the data to be transmitted.
Conventional semiconductor laser driver circuits are described in U.S. Pat. No. 5,883,910, issued Mar. 16, 1999 in the name of inventor G. N. Link and entitled xe2x80x9cHigh Speed Semiconductor Laser Driver Circuits,xe2x80x9d which is incorporated by reference herein.
When implementing a laser driver circuit in a high-speed application, e.g., a data transmission system operating at a rate of about 2.5 Gbits per second or more, Silicon-Germanium (SiGe) bipolar transistors are often used. The SiGe bipolar transistors can accommodate the high data rate while also saving power compared to standard bipolar or CMOS technologies. However, such SiGe transistors generally have a very thin base region, and as a result the base-emitter reverse bias must be limited to no more than about 1 volt in order to avoid damage to the base junction. This stringent base-emitter reverse bias constraint adversely impacts the design of interface circuitry, such as high-speed pseudo-ECL (PECL) input circuits used to interface data and clock inputs to the laser driver circuit input stage. More particularly, the PECL input specifications calls for inputs as large as 1.6 volts, which can violate the base-emitter reverse bias constraint in the laser driver input stage, particularly at higher temperatures. This in turn can lead to long-term performance degradation or other types of damage for the input stage transistors.
It should be noted that differential signaling is preferred in high-speed applications in order to reject ground noise and control output switching transients, thereby allowing smaller signal swings for a given bit error rate.
Current practice in addressing the base-emitter reverse bias constraint is to restrict the input differential voltage swing applied to the laser driver input stage to about 1.5 volts or less, or to provide input clamping circuitry. The former approach is undesirable in that it is inconsistent with the above-noted PECL specification and requires additional care in device testing and manufacturing. The clamping approach, while being acceptable for low-speed applications, can unduly limit the maximum operating speed of the driver, and is therefore impractical in high-speed applications.
It is therefore apparent that a need exists for improved optical source driver circuits which are configured to accommodate full differential input voltage swings without unduly impacting operating speed.
The invention provides improved optical source driver circuits which meet the above-noted need.
In accordance with one aspect of the invention, a driver circuit for a laser diode or other optical source includes an input stage, an output stage and a current generator circuit. The current generator circuit is adapted to establish a modulation current for application to one of a first output and a second output of the output stage in accordance with a differential input data signal applied to the input stage. The input stage includes first and second differential pairs. The first differential pair has the differential input data signal applied thereto, is implemented using MOS devices, and has substantially unity gain. The second differential pair receives as its inputs corresponding outputs of the first differential pair, is implemented using bipolar devices, and has a gain greater than unity. More particularly, the first and second differential pairs are configured such that application of the differential input data signal at a substantially rail-to-rail voltage swing to the first differential pair will not exceed a junction reverse bias constraint of the second differential pair.
Advantageously, the invention permits an optical source driver circuit to accommodate full differential input voltage swings, i.e., substantially rail-to-rail input voltage swings, while also maintaining the ability of the driver circuit to operate at high speeds.